Photo lithographically patterned spring structures (sometimes referred to as “micro-springs”) represent one type of micro-machined structure that has been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. Conventional spring structures include a spring metal finger (beam) having a flat anchor portion secured to a substrate, and a curved free portion extending from the anchor portion and bending away from the substrate (i.e., such that an air-gap is defined between the tip of the spring metal finger and the substrate to which the anchor portion is attached). The spring metal finger is formed from a stress-engineered metal film (i.e., a metal film fabricated such that its lower portions have a different internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The free portion of the spring metal finger bends away from the substrate when the release material located under the spring finger is etched away. The internal stress gradient is produced in the spring metal by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the spring is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring metal structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
The stress-engineered metal films used to form conventional spring structures were originally formed by sputtering deposition methods, but more recently plating deposition methods have been developed that produce suitable stress-engineered films. Those skilled in the art will appreciate the significant cost savings associated with using plating techniques, as opposed to sputter techniques, to fabricate the stress-engineered films. However, although modifying the spring production process to include plating the stress-engineered films reduces the overall costs significantly (i.e., no expensive stressed metal sputter machine needed), the existing technology still relies on depositing the release material by other methods such as sputtering (e.g., when titanium (Ti) is used as the release material) or plasma-enhanced-vapor-deposition (PECVD) (e.g., when silicon (Si) is used as the release material). Furthermore, a plating seed layer (e.g., Au) is typically required to facilitate the plating process, and this seed layer is typically sputter deposited over the release layer before stressed-metal plating. Thus, although the ability to form stress-engineered spring structures using plating deposition techniques reduces production costs, the need for expensive sputter deposition equipment is still required. Further, the ability to eliminate sputter deposition and to implement a plating-only production process is very difficult to achieve with the current spring materials due to the limited material choice (e.g., nickel (Ni), copper (Cu), gold (Au), nickel-phosphorous (NiP) alloy or nickel-boron (NiB) alloy) for plating, and associated etch selectivity problems. Note that fabrication costs are especially important in the targeted application areas such as packaging, probing and interconnects.
What is needed is a spring production method that utilizes plating deposition techniques to form the release layer, plating seed layer (when used), and the spring (e.g., stress-engineered metal) film.